1. Field of the Invention
This invention relates generally to distance measuring apparatus and more specifically, to an improved retroreflective prism assembly for use in conjunction with distance measuring instruments that employ infrared or laser ranging systems.
2. Prior Art
The last decade or so has seen the rapid development of microprocessors and infrared and laser optical systems with the resulting development of various electronics distance measuring systems for use in civil engineering applications including surveying. Usually such electronic distance measuring instruments consist of two main components, namely, the electronic ranging system that utilizes a transmmitter/receiver of either infrared or laser energy that is transmitted to a remotely and previously placed reflector assembly, and a sighting telescope that is used to accurately point the ranging system transmitter at the distantly located reflector. Consequently, the remotely located reflectors utilize retroreflector devices such as prisms or other such reflecting optics that reflect incident energy on a path that colinear with that instant energy. Even though retro-reflector devices or prisms are designed to return incident infrared or laser energy over a broad range of incoming angles with respect to the face of the reflector, the sensitivity of the receiver portion of the electronic distance measuring instruments usually requires that the reflector be adjusted to be substantially normal to the incoming energy. Consequently, it is desirable to have a retro-reflector device that is adjustable both in elevation and azimuth with respect to the tripod upon which the device is mounted. Unfortunately, prior art retro-reflective prism assemblies do not provide for tilting capability for electrical adjustment. As a result such prior art devices must be used in combination with a separate tiltable bracket to provide that additional needed capability for elevational adjustment. However, such additional brackets substantially increase the cost and inconvenience of utilizing such prior art retro-reflective prism assemblies and as a result, increase the overall cost and time consumed in carrying out the surveying process. In this respect, an additional disadvantage of the prior art is the lack of a convenient means for azimuth adjustment on the tripod. Typical prior art retro-reflective prism assemblies are screwed onto a threaded post projecting vertically from the top of the tripod. The tripod is first set in place at the desired location and the retro-reflective prism assembly may then be attached to the tripod at that time by inserting the tripod threaded extension into the base of the prism assembly and turning the prism assembly about a vertical axis until the assembly is screwed down firmly against the top of the tripod. Unfortunately, the azimuth relationship between the prism assembly and the tripod is typically fixed and as a result, in order to change the azimuth direction of the prism assembly it is necessary to pick up the entire tripod and prism assembly combination and rotate that combination about a vertical axis. Although this achieves the desired change in azimuth direction for the prism assembly, it also often results in a perceptible change in the actual physical location of the tripod and as a result, may cause inaccurate measurement or require additional time consuming processes to realign the various components of the measurement process.
Still an additional frequency requirement for retro-reflective prism assemblies is that they be plungable. The term plungable means rotatable approximately 180 degrees about a horizontal axis so that the prism can be redirected to point in substantially the opposite direction without moving the tripod. Obviously, prior art devices which are not tiltable are certainly not plungable and even those that are mechanically connected to tiltable bracket assemblies to enable elevation adjustment are often not plungable because those bracket assemblies are not designed for that degree of movement freedom.
There are numerous other deficiencies in the prior art which as will be seen hereinafter are remedied in the present invention. By way of example, even in those prior art devices that are provided with a separate tiltable bracket to enable elevation adjustment of the prism assembly, a tightening mechanism, such as a threaded knob, is used to secure the prism assembly in its selected elevational position and typically changes the elevational adjustment when it is being tightened. As a result, it is not uncommon for precisely selected elevational angles to be inadvertently changed when the elevation locking mechanism is tightened to secure the prism assembly in its tilted position. Therefore, it is very difficult to obtain a precise elevation angular adjustment selected for the prism assembly without compensating for the additional elevational change induced by tightening that knob. Furthermore, many of the known prior art devices which also utilize a prism consisting of a retro-reflective prism secure the prism within a housing in such a manner that the prism is either not well sealed against the elements such as dust, rain or other condensation or the prism is not well protected against shock. Furthermore it is not uncommon for prior art prisms to be installed in a housing in such a manner that in order to insulate the prism from any substantial levels of shock it is necessary to also place a resilient material behind the prism such as foam rubber. The lack of adequate sealing requires that the prism be coated on its rear reflecting surface in order to retain a consistent reflective coefficient which would otherwise be rendered inconsistent by dust, dirt or water. Unfortunately, such coating tends to reduce the reflective characteristics of the prism.